This invention pertains to impact and rotary rock drilling tools and in particular to down hole drill hammers such as disclosed in GB-PS No. 1,542,139.
Designs according to GB-PS No. 1,419,981 and DE-PS No. 23 62 724 incorporating, within an outer tube, an inner central tube that is reciprocating to plunge downwardly into the head of a steel drill member are disadvantageous in that the lower tool portions are subjected to heavy wear and tear resulting in the risk that the steel drill member may get stuck and/or break the lower central tube end. Also, splining provided for rotary slaving of the steel drill member shaft may deteriorate and get jammed under forces and torques which may become excessive when the inner tube is no longer in the center of the outer tube. In rock cavities suddenly reached, drilling may discontinue due to sagging of the steel drill member which is then actually cold-forged by the ram, with corresponding damage to retaining parts.
In a drill according to US-PS No. 3,970,153 there is a piston the lower end of which must slide into an outer tube guide each time, or else the percussion operation would neither get going nor continue. Since upwardly, too, the piston must find its way back onto the lower end of a central projecting tube member, the latter is prone to break under rough operating conditions.